An X-ray generator of a type in which an X-ray is generated by irradiating a target with an electron beam in a vacuum container uses a reflective type target that extracts an X-ray in a different direction from a traveling direction of an electron or a transmission type target that extracts an X-ray in substantially the same direction as a traveling direction of an electron. In the X-ray generator using the reflective type target, an X-ray focal spot diameter (a diameter of a region in which an X-ray is generated) depends on a focal spot diameter of an electron beam emitted on the target (a spot diameter of the electron beam emitted on a target surface) and a surface angle of the target with respect to the electron beam. On the other hand, in the X-ray generator using the transmission type target, an X-ray focal spot diameter is determined based only on a focal spot diameter of an electron beam emitted on the target.
A configuration example of the X-ray generator using the transmission type target is illustrated as a schematic cross-sectional view in FIG. 8. An X-ray irradiation window 101 is fixed to one end portion of a vacuum container 100, and a target 102 for generation of X-rays is stacked on a lower surface side (an inner surface side of the container) of the X-ray irradiation window 101. The X-ray irradiation window 101 and the target 102 are integrated to form an inseparable member, and included in a target stacked structure 103. An electron gun 104 including an electron source and an electrode group is accommodated in the vacuum container 100. Further, an X-ray generated by irradiating the target 102 with an accelerated and focused electron beam from the electron gun 104 is extracted in substantially the same direction as an irradiation direction of the electron beam B through the X-ray irradiation window 101. The term “X-ray irradiation window” is used since the X-ray is emitted from the X-ray generator through the member. However, the term is also referred to as a target substrate or simply as a substrate considering a function as a member for holding the target. In this specification, only the X-ray irradiation window is used among these terms.
FIG. 9 illustrates an enlarged view of a portion around a region in which the target 102 is irradiated with the electron beam B in FIG. 8 and a graph indicating an X-ray profile emitted to the outside by this configuration. The X-ray profile is expressed by a graph in which a horizontal axis represents a position and a vertical axis represents X-ray intensity.
As illustrated in FIG. 9, a focal spot diameter of the electron beam B with respect to the target 102, that is, an irradiation spot diameter of the electron beam B emitted onto a surface of the target 102 corresponds to a focal spot diameter of the X-ray in the X-ray generator. When this X-ray focal spot diameter is reduced, for example, the spatial resolution of a fluoroscopic image obtained by an X-ray fluoroscopic apparatus is improved, and thus a clearer image is obtained.
In this regard, conventionally, in the X-ray generator using the transmission type target, a method of narrowing an electron beam and emitting the electron beam onto the target has been adopted to reduce the X-ray focal spot diameter. However, it is extremely difficult to narrow the electron beam that spreads from an electron source due to a problem of an aberration of a lens that narrows the electron beam. As a countermeasure, a method has been frequently adopted to reduce an influence of the aberration by providing a beam aperture. However, when the electron beam is formed as small as submicron order, there arises a new problem that the X-ray focal spot diameter increases due to diffusion of electrons in the target.
In this regard, in general, a technology has been proposed to reduce the X-ray focal spot without narrowing the focal spot diameter of the electron beam emitted toward the target by adopting a structure in which a target such as tungsten stacked on one surface of an X-ray irradiation window in the form of a thin film is buried in an X-ray irradiation window made of light metal as a fine columnar metal wire (for example, see Patent Document 1), or by forming a fine columnar hole portion in an X-ray irradiation window to deposit metal corresponding to a target material in the hole portion (for example, see Patent Document 2).
That is, as a schematic cross-sectional view and a graph of an X-ray profile emitted to the outside by a target structure are illustrated in FIG. 10, a technology has been proposed to reduce an X-ray focal spot diameter without narrowing the focal spot diameter of the electron beam B emitted toward a target 202 by restricting an X-ray generation region and reducing an influence of electron diffusion in the target 202 using a structure in which the target 202 having a fine columnar shape is held in an X-ray irradiation window 201.